1. Capsular polysaccharides (CPS) of Streptococcus pneumoniae and coaggregation receptor polysaccharide (RPS) of S. oralis although closely related, are functionally distinct. CPS protects pathogenic S. pneumoniae from phogcytic killing whereas RPS of commensal S. oralis mediates interactions with other members of the dental plaque biofilm community. We recently completed comparative structural and molecular studies of ribitol-containing types of RPS (i.e. RPS4Gn and RPS5Gn) and closely related CPS (i.e. CPS10A, CPS10B, CPS10C and CPS10F). The results provide important insights into RPS structure, function and evolution. They also opened an approach for further comparative characterization of related S. pneumoniae serotypes, including pneumoniae CPS39. The structure of CPS39 was determined from sugar composition analysis performed at the Complex Carbohydrate Research Center in Georgia and high resolution NMR spectra. The repeating unit of this polysaccharide resembles those of CPS10A and RPS4Gn but contains arabitol rather than ribitol. From previous studies of CPS10A and RPS4Gn, we were able to assign each linkage in CPS39 to a gene in the cps39 locus. The only ambiguity involved the genes designated wcrC in the cps10A and cps39 loci. Whereas WcrC of CPS serotype 10A links Gal 1-2 to ribitol, the transferase encoded in the cps39 locus appears to link Gal 1-1 to arabitol. Thus, we suspect that the corresponding gene in the cps39 locus is not wcrC, but instead, a distinct gene. The goal of this work is to define the genetic basis of CPS and RPS structure. 2. The presence of Gn- or G-types of RPS on many strains of S. sanguinis, S. gordonii and S. oralis accounts for coaggregations noted between these bacteria and plaque bacteria such as Actinomyces spp. that have RPS-binding adhesins. In addition to Gn- or G-types of RPS, we suspect that other polysaccharides described from strains or S. oralis and S. mitis mediate interbacterial adhesion and thus, represent novel types of RPS. To test this hypothesis, we are preparing specific antibodies against the later polysaccharide for use as probes to identify the corresponding bacteria in naturally occurring oral biofilm communities. Following identification of these bacteria in vivo, we plan to isolate and characterize neighboring bacterial cells for the presence of RPS-binding complementary surface adhesins. These studies constitute a critical test of our hypothesis that a wide range of bacterial surface polysaccharides function as specific recognition molecules for dental plaque biofilm development and that RPS structure influences community composition. 3. Although saliva is a primary source of carbon and nitrogen for growth of different plaque species, little is known about how bacteria in biofilm communities degrade and utilize salivary proteins and glycoproteins for growth. We hypothesize that the close association of different cell types in dental plaque favors cooperative degradation of salivary proteins and glycoproteins by members of the biofilm community. To test this hypothesis, we are developing robust experimental models for studies of bacterial growth and biofilm formation in filter-sterilized whole saliva. In recent studies, the number of A. naeslundii T14V in co-cultures with S. oralis 34 was significantly greater than the number seen in actinomyces monocultures. Moreover, the actinomyces observed in co-cultures were closely associated with streptococci. In other experiments, we compared growth of different streptococci including strains of S. gordonii, S. oralis and S. mutans. In general, strains of S. gordonii and S. oralis grew well in saliva whereas strains of S. mutans grew poorly or not at all. Currently, we are exploring whether growth of S. gordonii and/or S. oralis in saliva promotes associated growth of S. mutans, a possibility that has important implications for initial colonization of cariogenic S. mutans in the absence of sucrose. Interestingly, S. gordonii DL1 and S. oralis Uo5, commensal species that grow well in saliva, appear to have a greater array of cell surface glycoside hydrolases than S. mutans UA159, which grows poorly or not at all in saliva. To assess the contribution of specific glycoside hydrolases for growth of S. gordonii DL1 and S. oralis Uo5 in saliva, we are introducing unmarked, in-frame deletions in genes predicted to encode these proteins. We anticipate that growth studies performed with well-characterized wild type and mutant strains, individually and in co-cultures, will provide new insight into the underlying basis of dental plaque development, opening new approaches for prevention, diagnosis or treatment of plaque-associated oral diseases.